In many applications, particularly in the field of power amplifiers for low and medium frequency signals, and primarily in the field of high efficiency audio amplifiers, a Pulse Width Modulation with switching frequencies of up to several hundreds of KHz is often a preferred approach. In these applications, the output power stage includes at least a power device switching the output to the supply line (HIGH SIDE DRIVER) and at least a complementary power device, switching the output to the ground line (Low SIDE DRIVER), driven by a digital PWM signal that may have a switching frequency of several hundreds of KHz.
PWM power stages are often realized monolithically on the same chip that contains the drive and control circuitry, and it is necessary to ensure that the turn-on of one or the other of the two power switching devices, respectively of one and the other switching branch of the output node, never happens before the other device has already started the turn-off. If so, this would increase the power dissipation in the superposition intervals and, because of the high switching frequency, could lead to the failure of the power devices.
To prevent such a destructive condition from occurring, a small time interval must be established, during which both power transistors of the output stage are turned-off. Such an interval is commonly referred to as "dead zone" or "dead time". In all the applications where it is necessary to ensure a high accuracy (such as in low distortion audio applications), the dead time should be minimized for obvious reasons.
Commonly, to avoid a simultaneous turn-on of the two branches of a output power stage, a dead zone for prevention of accidental overlapping of the turn-on commands for the two complementary devices of the stage is determined, by suitable delay circuits, based on RC networks dimensioned to ensure a certain delay, that is established by considering the worst condition of operation. In practice it is very difficult or almost impossible to fully optimize the delay value introduced in the turn-on commands because of the impossibility of effective electrical values of the manufacturing process of the integrated circuit, of their temperature dependence as well as of the variability of the load and other parameters that may affect it in a more or less marked manner.
The known solutions do not allow an optimization for minimizing the dead time value to be introduced in the turn-on signal of the complementary pair of power transistors of the output stage. moreover, in many applications, as in high efficiency audio amplifiers, in order to ensure most efficient driving conditions, switching from a bridge to single ended driving modes and vice versa is often implemented.